CD4 T cells play a pivotal role in the immune response primarily by directing responses of other lymphoid cells. Regulation occurs through cell-cell contact with antigen-presenting cells which directly induces signaling pathways leading to activation and function, and also results in cytokine secretion which governs the nature of the response. It is generally accepted that CD4 function in aged individuals is diminished, although the reasons and mechanisms responsible for this are not clear. Recognition of peptide/MHC complexes on APCs is a prerequisite for most aspects of CD4 function, however additional interactions between T cell co-receptors and APC accessory molecules are required for optimal cell growth, secretion of cytokines and induction of effector function. Thus, with aging, it is possible that T cells either do not express the appropriate co-receptors for efficient response, or are hypo-responsive to signaling through these molecules. An alternative, which is not mutually exclusive, is that defects exist in provision of accessory molecule help from APCs such as B cells and macrophages. The majority of studies suggest that CD28/B7, CD40L/CD40 and LFA-1/ICAM-1 interactions are critical for initial CD4 and APC activation in young individuals, whereas other molecules may be crucial for later phases of primary responses and potentially for promoting secondary responses involving memory T cells. Both already published data and our unpublished studies suggest that two ligand-receptor pairs, which are primarily expressed on T cells and APC after their initial activation, have co- stimulatory activities which can be equivalent but also distinct from the molecules above. These are members of the TNF/TNFR families, namely Ox-40- Ox-40L and 4-1BB-4-1BBL, and as such represent potential targets for defects associated with aging. The studies proposed here will assess the function and roles of these molecules in T cell and APC responses from young versus old mice. In vitro experiments will analyze naive and memory CD4 cells and assess expression of OX-40 and 4-1BB with age and whether conditions for expression of these molecules changes. We will additionally assess T cell proliferative and cytokine responses induced in responses to anti-CD3 using antibodies or Fc constructs which can ligate Ox-40 and 4- 1BB. These studies will be complemented by assessing induction and expression of Ox-40L and 4-1BBL on B cells and macrophages from aged mice, and whether cross-linking these molecules on APC is defective for inducing B cell and macrophage function compared to cells from young animals. These studies will provide novel findings with regard to T cell-APC interactions with age and may highlight ways in which hyporesponsiveness of these cells can be corrected.